In recent years, broad band access, such as Fiber To The Home (FTTH) has been expanding and progressing. In current in-home wiring arrangements, a UTP cable or a wireless setup is usually used, but in order to achieve greater broadband transmission, direct connection of optical fiber in offices and homes is being considered.
The allowable bend radius for conventional single-mode optical fiber, as defined by ITU-T G652, is a minimum of 30 mm. If a bend radius less than this is applied, the amount of loss is increased and the transmission signal decays.
When considering wiring in homes, which can involve running the optical fiber along a wall and often subjects the optical fiber to rough treatment, it is unrealistic to arrange the wiring in a manner that such a large curvature radius is always maintained. Therefore, single-mode optical fiber is desired with good flexibility, sufficient for preventing an increase in loss due to a smaller bend radius, and such optical fiber is actually being developed and sold. Here, “flexible” means that the optical fiber can bend without incurring a significant increase in transmission loss.
In apparatuses used in the offices of telecommunication providers, flexible single-mode optical fiber allowing for a smaller bend radius is used to make these apparatuses more compact.
The flexible single-mode optical fiber can be realized by simply increasing the refractive index of the core in the conventional single-mode optical fiber. In this case, however, the mode field diameter is decreased. Therefore, there is a problem of increased connection loss with standard single-mode optical fiber.
Japanese Patent No. 3,853,833 and Fujikura Technical Review No. 105 (pp. 6-10) disclose flexible single-mode optical fiber to solve the above problem. This single mode optical fiber has high flexibility and a mode field diameter that is close to that of standard single-mode optical fiber. As shown in FIG. 1, the refractive index distribution of this optical fiber is made up of four layers including a core 100, a first cladding 101, a second cladding 102, and a third cladding 103, and when the refractive indexes of these layers are respectively set to n0, n1, n2, and n3, the core is doped with germanium until n0 becomes greater than n3. Furthermore, the second cladding 102 is doped with fluorine until n2 becomes less than n3. The third cladding 103 is formed of pure quartz. The first cladding 101 is doped with a dopant to raise or lower the refractive index thereof as needed.
Japanese Patent Application Publication No. 2007-279739 also discloses a flexible single-mode optical fiber, but this optical fiber has a refractive index distribution that is similar to that of the optical fiber disclosed in Japanese Patent No. 3,853,833. Japanese Patent Application Publication No. 2007-45643 discloses a method for manufacturing a pure silica core fiber that has low OH content, using a high-frequency induction thermal plasma torch.